We have an ecobee thermostat hooked up to it and love it. 

Tenants had thermostats in their office suites that they had set at 68 degrees. 

If you have thermostats where you can limit temp setting that will save you.Electric LED lights/bulbs. 

I framed in an additional shower, replaced 4000 square feet of flooring with LVP to hold up to rigors of STR, removed 50 gallon electric water heater and replaced with a 75 gallon LP unit, completely renovated both sets of stairs, added additional railings to bring up to code, new trim throughout the house, added automation with thermostats and locks, and also furnished it via secondhand stores with some new items. 

Operating electronics (Wifi password, thermostat settings, TV) 3. 

The only thing I didn't quite get about your post was the "upstairs unit controlling temperature of downstairs unit" part - are you saying that there's one thermostat in this building for 4 units? 

All units have their own thermostat. 

Cadet makes a nice small in wall heater 220v 1000watts; the thermostat is sold separately so watch for that, amazingly stupid I know.

Once I verified everything is good working condition then I would suggest to the tenant they be put on a budget plan with the gas company and adjust the thermostat setpoint to save some money. 

On Monday, she complained that the programmable thermostat was broken.